Fluid samplers are regularly used to sample gases, particularly air, to determine the degree of exposure to hazardous chemicals. A typical sampling method involves collecting a sample of test gas, such as air, by drawing a known volume of the test gas through a collecting device. The collecting device may include a solid adsorbent (sorbent tube) capable of trapping and removing chemicals from the air or may include a filter for selectively collecting particulates, or a combination of both. This method is also applicable to wet test sampling referred to in the art as "impinger sampling." The test sample is then analyzed to determine the concentration level of the collected sample of chemicals or particulate matter. The method of analysis may involve gas chromatography or atomic adsorption, etc. For the analysis to be accurate, it is necessary to control the rate of fluid flow through the collecting device and it is preferable to use a calibrated flow rate.
All known pump sampling systems which control flow by adjustment of pump motor speed produce a pulsed air flow with relatively high pulse undulations at low flow levels. With a highly pulsed flow, it is difficult to set the flow rate. In fact, at very low flow rates, most common flow meters cannot be used to accurately determine or calibrate the flow because of their sensitivity, in part, to pulse magnitude as well as to actual net flow. High pulse undulations also tend to generate undersirable audible noise levels.
To minimize the amplitude of the pulse undulations at low flow rates and to reduce the noise level, a double-acting pump was typically used in conjunction with at least one damping assembly on the input suction side of the pump. A typical double-acting pump for contolling fluid flow in a fluid sampler over a wide range of flow rates is described in U.S. Pat. No. 4,432,248, assigned to the common assignee, Gilian Instrument Corporation, the disclosure of which is incorporated herein by reference. The fluid sampler incorporates two damping assemblies mounted in a common body with the double-acting pump. The damping assemblies communicate with the pump through an air port which connects the damper volumes to the pump intake. The double-acting pump and an electronically controlled pump motor speed control provides the fluid sampler described in the aforementioned patent with a regulated and smooth flow response over an operating pump flow range between 500 milliliters and 4 liters. Below 500 milliliters, the flow is unregulated, although still relatively smooth and noiseless.
For applications where it is important that the fluid sampler be very small in size as well as light in weight, a high efficiency, single-acting pump is preferable to the double-acting pump. A single-acting pump contains only one valve plate with one inlet valve and one exhaust valve. Accordingly, a fluid sampler with a single-acting pump draws fluid through a load on the intake stroke of the pump when the exhaust valve is closed and the intake valve is open. Conversely, fluid is discharged only during the exhaust stroke of the pump with the exhaust valve open and the intake valve closed. Thus with only one inlet valve and one exhaust valve, the single-acting pump at low flow rates will inherently cause large pulse swings. Although the large undulations in flow may be reduced by means of a damper located at the input to the pump, the flow response will not be as smooth in flow characteristic to that of the double-acting pump.